It has been tried to use GaN-based semiconductors made of compounds of the III group element(s) such as Al, Ga and In with the V group element of N as semiconductors for light emitting devices or power devices, because of their favorable energy band structures and chemical stability. For example, production of blue semiconductor laser devices by stacking a plurality of GaN-based semiconductor layers on each of sapphire or GaN substrates has been attempted vigorously.
An example of a bluish green semiconductor laser device is shown in FIG. 23 in schematic perspective view, wherein a ridge-type waveguide is formed to cause a refractive index difference in a direction parallel to the semiconductor junction surface, to thereby confine light for lasing (see, e.g., Jpn. J. Appl. Phys., Vol. 37 (1998) pp. L309–L312 and Jpn. J. Appl. Phys., Vol. 39 (2000) pp. L647–L650).
In the GaN-based semiconductor laser 1100 of FIG. 23, a GaN thick film is formed on a (0001) plane sapphire substrate (not shown). After removal of the sapphire substrate, the GaN thick film is used as the (0001) plane GaN substrate 501. Stacked successively on GaN substrate 501 are a GaN buffer layer 502, an n-type GaN contact layer 503, an n-type AlGaN clad layer 504, an n-type GaN guide layer 505, a multiple quantum well active layer 506 utilizing InGaN, a p-type AlGaN evaporation preventing layer 507, a p-type GaN guide layer 508, a p-type AlGaN clad layer 509, and a p-type GaN contact layer 510.
In this semiconductor laser 1100, a ridge stripe 511 is formed with an an upper portion of p-type AlGaN clad layer 509 and p-type GaN contact layer 510. As such, a stripe-shaped waveguide for confinement of a lateral transverse mode is provided by creating the refractive index difference in the direction parallel to the semiconductor junction surface.
A SiO2 dielectric film 513 not absorbing the light from active layer 506 is formed on each side surface of ridge stripe 511 so as to form a current-constricting structure for introducing electric current only from the top surface of the ridge stripe. The refractive index at ridge stripe portion 511 is higher than that at either side thereof, so that a refractive index distribution in a mesa-like shape is created in the direction parallel to the semiconductor junction surface.
A p-side electrode 515 is formed on the top surface of ridge stripe 511 and on the upper surface of SiO2 dielectric film 513. Further, an n-side electrode 517 is deposited on n-type GaN contact layer 503 having been partially exposed by reactive ion etching (RIE). These electrodes serve to introduce the current into semiconductor laser 1100.
In semiconductor laser 1100, mirror facets are formed by dry etching and the light confinement is achieved by the mesa-like refractive index distribution in ridge stripe portion 511, so that it is possible to obtain stable lasing of the lateral transverse mode with a low threshold current. Furthermore, the lifetime of that semiconductor laser exceeds 10,000 hours, and thus it is considered the semiconductor laser technology has almost been completed in terms of the long life and accompanying reliability of the laser.
However, when lasing is kept up to a high optical output region in the laser having the structure as shown in FIG. 23, linearity of the current and optical output (I-L) characteristic may be impaired during the process of increasing the amount of introduced current. It is known that there are cases where the optical output becomes out of proportion to increase of the current, causing a stepped change of the optical output that is called a “kink”. Such a kink occurring in a laser device involves a sift in the emitting direction of the laser beam as well as fluctuation of the output, thereby causing critical problems in practical use of the laser. It is considered that the kink has a close relation with stability of the lateral transverse mode. The following are two conceivable reasons for occurrence of the kink.
Firstly, in an InxGa1·xN (0≦x≦1) crystal that is often used for the active layer of the GaN-based laser, regions having different In composition ratios are liable to be formed, which may cause localization of carriers and hence occurrence of the kink. Secondly, the effective mass of the carriers in the GaN-based material is large, so that localization of the carriers may occur and then it causes the kink. Thus, in order to prevent such occurrence of the kink, the GaN-based semiconductor requires laser design different from that with the other semiconductor materials, in consideration of its particular physical properties.
Generally, to prevent the occurrence of the kink, it is effective to narrow the width of the stripe-shaped waveguide. When the stripe width is narrowed, however, the width of the current path is narrowed as well, which causes increase of the operation voltage and then generation of heat, thereby leading to decrease of the lifetime and reliability of the laser. As such, it is desired to provide a structure ensuring stability of the transverse mode, with the minimum necessity of narrowing the current path.
In view of the foregoing, an object of the present invention is to solve the above-described problems, so as to provide GaN-based laser devices that can suitably be used for optical pickups or the like, with a good yield rate.